1. Field of the Invention
The present invention relates to a developing roller comprising a semiconductive rubber for use in an electrostatic recording apparatus, and to an electrostatic recording apparatus using the developing roller. More particularly, the present invention relates to a developing roller for use in an electrostatic recording apparatus comprising an electrographic imaging apparatus, where the developing roller comprises a conductive carbon black, a conductive polymer and a binder resin. The invention is also directed to an electrographic imaging apparatus including the developing roller.
2. Description of the Related Art
Conventionally, an electrographic imaging apparatus for an electrographic method comprises a photoreceptor on which a digital image signal is received by exposing a region to form an electrostatic latent image, and a developing roller that applies a developing agent (hereinafter is referred to a toner) to the electrostatic latent image. The photoreceptor and the developing roller are rotated while in engagement with each other. One side of the developing roller includes a means for feeding a toner onto the developing roller. A transferring roller on a lower side of the photoreceptor is rotated while in engagement with the photoreceptor. A toner applied onto an electrostatic latent image of the photoreceptor is transferred to the transferring roller and then onto a recording medium passing through the photoreceptor and the developing roller to form an image.
The developing roller plays an important role in transferring a toner to a latent image on a photoreceptor to form a visible image. The developing roller must be prepared such that it has specific properties, in order to apply a toner smoothly and in proper amounts on a photoreceptor.
For example, a developing roller must have a small variation in physico-chemical properties to prevent the additives, etc. contained in a toner from separating from the toner and penetrating into the developing roller during charging of the toner and when friction is applied to the toner. Alternatively, a developing roller must have a small variation in electrical properties according to environmental changes to maintain the proper volume resistance and the necessary dielectric constant, and particularly to form an excellent image uniformly for a long time. Further, the filming phenomenon must not occur on the surface of the developing roller even after friction is applied for a long time with a toner.
A developing roller used in such electrostatic recording apparatus, etc. can be categorized into an ion conductive developing roller and an electron conductive developing roller.
An ion conductive developing roller uses a base rubber consisting of rubbers having a low volume resistance, such as a conductive hydrine-based rubber, in order to decrease the resistance of the roller. The base rubber can include conductive additives, for example, conductive polymer materials such as an ethylchlorohydrine, or ion conductive materials such as Na, Zn, K, Cu, Co or Ni. However, the rubber having a low resistance, such as a hydrine-based rubber, etc., used in the ion conductive developing roller is so expensive that it can be a factor in increasing the cost for producing a developing roller. Furthermore, a conductive additive such as an ion conductive material, which needs to be used in order to obtain satisfactory conductivity, changes molecular structures of a developing roller into a state that readily absorbs moisture. Thus, large variations in a volume resistance of the developing roller can occur as a resulting environmental change.
On the other hand, an electron conductive developing roller uses a carbon black or a metal powder such as copper in order to impart conductivity to a common base rubber. Accordingly, although an expensive conductive rubber having a low resistance is not used and the resistance according to environment little varies, the hardness of a roller can be increased by adding large amounts of a conductive carbon black, the resistance can become non-uniform as a result of poor dispersion of the carbon black or metal powder, and the roller surface is stained with a carbon black.
Research concerning such electron conductive developing roller has steadily progressed. U.S. Pat. No. 5,565,968 discloses a developing roller comprising a polyurethane, a nitrile rubber, a chloroprene rubber or butyl rubber, and the like, as the base rubber, and a carbon black and a conductive polymer. U.S. Pat. No. 6,393,243 discloses a developing roller that has a multi-layer structure consisting of an elastic layer and a resin layer, and uses a conductive polymer such as a polyacetylene, and the like. On the other hand, U.S. Pat. No. 6,458,883 discloses a developing roller that uses a mixture of a crosslinking polymer such as a butadiene rubber, a styrene-butadiene rubber, and a non-crosslinking polymer such as a nitrilebutadiene rubber as a base rubber. The roller also comprises a conductivity imparting agent and an ion conductivity imparting agent such as LiCF3SO3, and the like.
However, the hardness, the stability to resistance, the resistance to migration, etc. of the developing roller are not satisfactory for an actual electrographic imaging apparatus. In particular, when the developing roller comprises a conductive polymer and a carbon black, after extended use the conductive polymer material or the carbon black can migrate to the surface of the developing roller, thereby contaminating the image. Thus, a developing roller that can form improved images uniformly for a long period is required.